The effects of aging upon animal models of pain perception have largely centered upon changes in basal pain thresholds and in morphine analgesia. The recent use of environmental and stressful manipulations has indicated that opioid and non-opioid pain-inhibitory systems exist in young adult rodents. Since aging animals display impaired hypothelamo-hypophysial-adrenal responses to stress and since alterations in these systems affect opioid and non-opioid pain inhibition differentially, the present proposal has two major goals: a) assess whether aging or aged rats display differential impairments in their analgesic responses to either opioid (central administration of the mu-receptor agonist, beta-endorphin or the delta-receptor agonist, D-ala2-D-leu5-enkephalin), opioid-mediated (central administration of substance P, acute exposure to either 2-deoxy-D-glucose or prolonged intermittent foot shock), or non-opioid (central administration of vasopressin or neurotensin, acute exposure to either cold-water swims or brief, continuous foot shock) manipulations; and b) gain insight regarding opioid and non-opioid pain-inhibitory substrates in aged populations that exhibit known physiological and biochemical deficits.